It is known that in the drying section or portion of a paper making machine the moisture-bearing paper web is directed along a serpentine path in wrapping relation with drying cylinders or drums arranged as in tiers and having the cylinders in one tier staggered with respect to the cylinders or drums of the other tier. In such dryer sections it is not unusual to have a plurality of such drying cylinders or drums arranged in such tiers with the tiers (or rows) being arranged in respectively different elevations.
Generally, at the beginning of such a dryer section a support band or belt is provided which meanders over the drying cylinders or drums. Generally, the support band or belt means may be a porous, air-permeable endless support band or belt of a felt generally woven from cotton or a plastic or wire, or fabric belt. Quite often in such dryer sections, the drying cylinders of, for example, the lower-situated tier actually have the support band or belt means running directly against the outer periphery thereof while the wet paper web, carried by the support band means, is actually radially outwardly and against the radially outer most surface means of the support belt or band means. In contrast, the drying cylinders of, for example, the upper-situated tier actually have the paper web running directly against the outer periphery thereof and effectively, runningly, held thereagainst by the support belt or band means which is situated directly radially outwardly of and against the paper web to be dried.
The support band means, together with the moist paper web which possesses little strength at that time, runs through the initial region of the dryer section and in so doing the support band means supports the paper web and does so in particular at the free and otherwise unsupported stretches or runs between pairs of successive drying cylinders or drums. Again, as already pointed-out, the case of the assumed relatively upper tier drying cylinders, herein for ease of reference also referred to as outward cylinders or outward cylinder, the support band or belt means effectively, in running fashion, holds the paper web directly against the outer drying surface of the associated drying cylinder while, in contrast, in the case of the assumed relatively lower tier drying cylinders, herein for ease of reference also referred to as inward cylinders or inward cylinder, the running support band or belt means actually lies against the outer surface of the associated drying cylinder and between such outer surface of the drying cylinder and the paper web.
In paper making machines having an especially high operating speed, as for example surface speeds greater than 850 meters per minute, difficulties occur in guiding the paper web in the dryer section. In fact, it has been discovered that the paper web lifts off the support band means as it runs off the surface of an outward drying cylinder. The air consequently drawn-in between the paper web and the running support band or belt means often prevents contact between the paper web and the support band or belt means in the further movement or conduction of the paper web in its desired normal course. This, in turn, leads to an unstable running of the paper web exemplified as by a wagging, fluttering and even flapping-over of paper web edge. As the paper web runs generally onto the following inward drying cylinder, air is again forced between the paper web and the running support band means. Consequently, as the paper web passes generally about such inward drying cylinder, the air thusly forced between the paper web and the support band, causes the paper web to remain lifted off the support band so that paper web conduction or transportation becomes unstable in such areas. Such an undesirable action of the paper web results in a considerable reduction in the quality of the paper and an increased risk of tearing the paper during its progress into and through the dryer section.
The prior art as illustrated by, for example, International Publication Number WO81/01428 (under the Patent Cooperation Treaty) employs an air guide suction box or apparatus 13.sup.111 (FIG. 6) which extends as by flat wall means along the support band between the outlet gore (or gore-like space) of an external or an outward drying cylinder and the inlet gore of an internal or inward drying cylinder. A second wall means of the suction box 13.sup.111 extends along the free surface of the internal or inward drying cylinder. At the inflow-side edge of these walls as well as in the inlet gore, the guide box 13.sup.111 is provided with air wipe-off strips; such strips are intended to suppress to a large extent the infiltration of air into gaps, at the support band and at the cylinder surface, defined by the two walls of the box. Also operatively connected to the box 13.sup.111 is a source of suction or vacuum 15 and such source communicates with openings both at the inflow-side edge of the box wall extending along the support band, 12, and at the gore-side edge of the wall associated with the drying cylinder. In this manner, the prior art intends to draw-off the air which despite the air wipe-off strips has been carried by the support band or belt means and by the cylinder surface into the respective gaps. Additionally, the gap extending along the support band, which gap is defined on the inflow and gore sides by additional air wipe-off strips directed against the support band, communicates with the atmosphere in the dryer section through channels discharging at the end toward the inlet gore. With such a prior art arrangement, it is hoped that the paper web will adhere to the support band or belt as in the area between two successive drying cylinders. However, apparatus conforming to such prior art teachings have not been found to be satisfactory. For example, it has been found that in such a prior art structure, the air will flow out of the gap on the support band side through the narrow channels of the box but only in a very small and insufficient quantity or rate. Further, such prior art structures, and in particular the air or suction boxes, are operationally unsafe because there is the danger that dust particles and fibers drawn-in with the drying air will soon obstruct or clog the suction or vacuum openings of the box 13.sup.111 and/or its flow paths.
The prior art as illustrated by Publication No. De-3148578 A1 of the Patent Office of the Federal Republic of Germany, employs, as shown in its FIGS. 6 and 8, air guide boxes where by the use of blast air the lifting-off of the paper web from the cooperating support band or belt in the region of the inlet gore is hopefully to be overcome and eliminated. To this end, in one form of the apparatus disclosed by said No. DE-3148578 A1, the air guide boxes are arranged directly before the inlet gore and blast air is ejected or directed both counter to the running direction of the support band means and counter to the direction of rotation of the adjacent drying cylinder. In another form or embodiment, of the teachings of said DE-3148578 A1, blast air is used only at the support band while an air wipe-off strip is directed against the free surface of the associated drying cylinder. The blast air as well as the air wipe-off strip each have the same purpose and that is to wipe off the air boundary layer entrained by the support band and/or by the drying cylinder and to prevent such air from penetrating into the inlet gore. However, tests and experience has shown that such structures embodying the teachings of said No. DE-3148578 A1 are less than acceptable. For example, it has been found that the paper web adheres against the support band but that such occurs only in a narrowly defined region along the band-side blast air ejection. Air having entered the inlet gore, despite the air guide box, causes a lifting-off of the paper web from the support belt or band. Further, because the surface of the support band presents a considerable resistance to the blast air stream, a relatively very high rate of blast air is required which, in and of itself is undesirable. However, as a further disadvantage, the provision of such a high rate of blast air often affects the heat balance of the dryer section in an undesirable manner.
The invention as herein disclosed and described is primarily directed to the solution of the foregoing as well as other related and attendant problems of the prior art.